1. Field of Invention
The present invention relates to a driving method for an electro-optical device, in which gray scale display is performed by temporal modulation, a driving circuit therefor, an electro-optical device, and electronic apparatus.
2. Description of Related Art
Electro-optical devices, such as liquid crystal display devices that use liquid crystal as an electro-optical material, have been widely used. Most commonly, electro-optical devices are used as an alternative display devices to cathode-ray tubes (CRTs), as display units of various kinds of information processing apparatuses, such as liquid crystal television sets, and the like.
Conventional electro-optical device can be formed of, for example, an element substrate incorporating pixel electrodes which are arranged into a matrix, switching elements coupled with the pixel electrodes, etc., an opposing substrate having counter electrodes formed thereon which face the pixel electrodes, and liquid crystal as an electro-optical material which is filled between the two substrates. In such a structure, when a single scanning line is selected, the switching elements become conductive. As an image signal having a voltage corresponding to a gray scale level is applied to the pixel electrodes via a data line while they are conductive, an electric charge corresponding to the voltage of the image signal is stored in the liquid crystal layer between the pixel electrodes and the counter electrodes. After the electric charge has been stored, the electric charge stored in the liquid crystal layer is maintained due to a capacitive property of the liquid crystal layer itself, a storage capacitance, and the like, even if the switching elements are turned off. Accordingly, when the switching elements are driven to control the amount of electric charge to be stored according to the gray scale level, the orientation of the liquid crystal changes at each pixel. This causes the density to change for each pixel, thereby achieving gray scale display.
In this regard, the electric charge need only be stored in the liquid crystal layer of each pixel for some period of time, and the structure which involves, first, sequentially selecting scanning lines, and, second, for a pixel intersecting the selected scanning line, applying an image signal having a voltage corresponding to the gray scale level of that pixel to the corresponding data line allows for time-division multiplexing which allows a scanning line and a data line to be commonly used for a plurality of pixels.
However, the image signal to be applied to a data line has a voltage corresponding to the gray scale level of a pixel, that is, an analog signal. This requires a D/A converting circuit, an op amp, etc., as peripheral circuits of the electro-optical device, leading to increased cost of the overall device. In addition, display nonuniformity is caused by unevenness in characteristics of the D/A converting circuit, the op amp, etc., in various wiring resistances, etc., leading to a problem in that it is extremely difficult to perform high-quality display. This is particularly significant for high-definition display.
There are also problems of increased power consumption resulting from the D/A converting circuit, op amp, etc.
The present invention has been made in view of the foregoing situation, and it is an object thereof to provide an electro-optical device capable of high-quality and high-definition gray scale display and reduced power consumption, a driving method therefor, and a driving circuit therefor, and further, to provide electronic apparatus incorporating the electro-optical device.
In order to achieve the above object, the present invention can include a pixel which is arranged corresponding to each intersection between a plurality of scanning lines and a plurality of data lines. The pixel can be turned on or off by the subfield, which is the unit obtained by dividing one field into subfields, according to weighting of gray scale data indicating the gray scale level of the pixel. Further, a reference time of weighting for the gray scale data can be shifted every scanning line and every subfield.
According to the invention, the period when a pixel is turned on or off in one field is subjected to temporal modulation (also called pulse width modulation) according to gray scale data indicating the gray scale level of that pixel, with the result that it is displayed in gray scale under effective value control. In this regard, a pixel is merely turned on or off in each subfield, and thus only required is data (that is, a digital signal that can only take low or high level) for an instruction signal to the pixel, thereby eliminating a processing circuit for analog signals. According to the first invention, therefore, there is no need for a D/A converting circuit, an op amp or the like, and, in addition, it is possible to suppress display nonuniformity resulting from unevenness in characteristic of these circuit elements or in various wiring resistances, etc. The power consumption can further be reduced.
According to the first invention, furthermore, a reference time of weighting for gray scale data is shifted every scanning line and every subfield, and it is not necessary to sequentially select all scanning lines in each subfield, but it is sufficient to select only the scanning line in which the reference time of weighting has arrived. This makes it possible to reduce the data transfer rate in one subfield.
As used herein, the reference time of weighting for gray scale data indicates, as shown in FIG. 7, when one field 1f is divided into subfields sf1 to sf17 and when allocated to each bit of gray scale data indicating the gray scale level of a pixel is a subfield number corresponding to a pulse width period according to weighting of the gray scale level indicated by that bit, for example, a timing of the start of each allocated period. Herein, when the gray scale level of a pixel is indicated, binary gray scale data is always used for the indication, however, that gray scale data and the actual displayed gray scale level may not be sometimes in a one-to-one relation (for example, even if gray scale data has four bits, only eight-gray scale level display may be possibly performed by ignoring particular bits). Alternatively, as described below with respect to the mode for carrying out the invention, a subfield may be sometimes allocated to correction bit h other than the gray scale data. Thus, it is just expressed herein as a reference time of weighting for gray scale data.
Furthermore, in the present invention, one field can mean the time period required to form a single raster image by performing horizontally and vertically scanning in synchronization with horizontal scan signals and vertical scan signals. Therefore, one frame according to the non-interlace method, etc., also corresponds to one field according to the present invention.
According to the invention, the order in which scanning lines are selected differs from one subfield to another, and the period when a pixel is turned on or off may also occasionally differ from one scanning line to another if scanning line in which the reference time of weighting has arrived is simply selected in order. According to the invention, therefore, preferably, while scanning line in which the reference time of weighting has arrived is selected in a predetermined order, selection of a single scanning line in a particular subfield and selection of the scanning line adjacent thereto in the next subfield are performed in the identically numbered horizontal scan period. This way can make the period when a pixel is turned on or off uniform in (pixels positioned on) each scanning line.
Making the on- or off-period uniform in this way may also be possible in the manner that every predetermined number of scanning lines are grouped into a block, each of the blocks are selected in a predetermined order in each subfield, and the scanning line in which the reference time of weighting has arrived is selected in a predetermined order within a selected block, while selection of a single scanning line in a particular subfield and selection of the scanning line adjacent thereto in the next subfield are performed in the identically numbered horizontal scan period.
Next, in order to achieve the above object, the invention can include a driving circuit for an electro-optical device, wherein a pixel which is arranged corresponding to each intersection between a plurality of scanning lines and a plurality of data lines is turned on or off by the subfield, which is the unit obtained by dividing one field into subfields, according to weighting of gray scale data indicating the gray scale level of the pixel, and a reference time of weighting for the gray scale data is shifted every scanning line and every subfield. The driving circuit can include a scanning line driving circuit for selecting the scanning line in which the reference time of weighting has arrived in a predetermined order in each subfield, and a data line driving circuit for supplying data to the pixel that intersects the scanning line selected by the scanning line driving circuit via the corresponding data line, the data indicating that the pixel is turned on or off. Again, the invention suppresses display nonuniformity resulting from unevenness to perform high-quality and high-definition gray scale display, and reduces the data transfer rate in one subfield.
In addition, in order to achieve the above object, the present invention can include an electro-optical device in which a pixel includes a switching element arranged corresponding to each intersection between a plurality of scanning lines and a plurality of data lines and a pixel electrode connected to the switching element, the pixel being turned on or off by the subfield, which is the unit obtained by dividing one field into subfields, according to weighting of gray scale data indicating the gray scale level of the pixel, and a reference time of weighting for the gray scale data is shifted every scanning line and every subfield. Also, the electro-optical device can include a scanning line driving circuit for selecting the scanning line in which the reference time of weighting has arrived in a predetermined order in each subfield, and a data line driving circuit for supplying data to the pixel that intersects the scanning line selected by the scanning line driving circuit via the corresponding data line, the data indicating that the pixel is turned on or off. Again, the invention suppresses display nonuniformity resulting from unevenness to perform high-quality and high-definition gray scale display, and reduces the data transfer rate in one subfield.
In the invention, when a DC component is applied to an electro-optical material interposed between a pixel electrode and a counter electrode, the electro-optical material may be occasionally deteriorated, so that the structure in which the voltage level to be applied to the counter electrode is inverted at intervals of a predetermined period and the voltage of data indicating that the pixel is turned on or off is inverted according to this inversion with reference to the voltage level applied to the counter electrode. Alternatively, the structure in which the voltage level applied to the counter electrode is constant and the voltage of data indicating that the pixel is turned on or off is inverted at intervals of a predetermined period with reference to the voltage level applied to the counter electrode is preferable.
Furthermore, in order to achieve the above object, the embodiments of the present invention can include the above-described electro-optical device, thereby making it possible to suppress display nonuniformity resulting from unevenness to achieve high-quality and high-definition gray scale display, and to reduce the data transfer rate in one subfield.